WIP: root background loops at context.Background() (refs #97)

Incomplete: tests pass but three lint findings remain (funcorder on
registerHooks, unparam in engine_integration_test.go). Committed to
preserve work in progress; to be amended into a single clean commit.

internal/delivery/engine_integration_test.go

@@ -476,7 +476,7 @@ func TestWorkerLifecycle_StartStop(t *testing.T) {
 	t.Parallel()
 
 	s := newISetup(t)
-	s.Engine.ExportStart(context.Background())
+	s.Engine.ExportStart()
 
 	event := iSeedEvent(
 		t, s.WebhookDB, s.WebhookID,
@@ -558,7 +558,7 @@ func TestWorkerLifecycle_ProcessesRetryChannel(
 		database.DeliveryStatusRetrying,
 	)
 
-	s.Engine.ExportStart(context.Background())
+	s.Engine.ExportStart()
 
 	bodyStr := event.Body
 	cfg := iHTTPConfig(ts.URL)